Offshore pipeline installation method

ABSTRACT

An underwater pipeline is installed by lowering it to the bottom of the water from the stern of a lay barge as the barge advances along a long preassembled pipeline section which floats near the surface of the water and is held in tension by a second vessel positioned in front of the lay barge. An additional floating section is connected in place when the lay barge reaches the end of the initial section, this additional section is held in tension by the second vessel, and laying of the line is continued as the lay barge advances.

United States Patent Matthews, Jr.

[ 1 Sept. 12, 1972 [54] OFFSHORE PIPELINE INSTALLATION METHOD [72]Inventor: Jamie F. Matthews, Jr., Houston,

Tex. 77024 [73] Assignee: Esso Production Company [22] Filed: Feb. 9,1971 [21 Appl. No.: 113,926

[52] US. Cl ..61/72.3 [51] Int. Cl. ..Fl6l 1/00, B63b 35/04 [58] Fieldof Search ..61/723, 72.1, 72.4

[56] References Cited UNITED STATES PATENTS 2,215,460 9/1940 Childress..6l/72.3 X 3,431,739 3/1969 Richardson et a1 ..61/72.3

Primary Examiner-Jacob Shapiro Attorney-James A. Reilly, John B.Davidson, Lewis H. Eatherton, James E. Gilchrist, Robert L. Graham andJames E. Reed [5 7] ABSTRACT An underwater pipeline is installed bylowering it to the bottom of the water from the stern of a lay barge asthe barge advances along a long preassembled pipeline section whichfloats near the surface of the water and is held in tension by a secondvessel positioned in front of the lay barge. An additional floatingsection is connected in place when the lay barge reaches the end of theinitial section, this additional section is held in tension by thesecond vessel, and laying of the line is continued as the lay bargeadvances.

12 Claims, 5 Drawing Figures PATENTEDSEP 12 I972 SHEET 2 OF 2 IN V1211"!UP JAMIE F MATTHEWS, JR.

A T TOR/VEY OFFSHORE PIPELINE INSTALLATION METHOD BACKGROUND OF THEINVENTION l. Field of the Invention This invention relates to the layingof underwater pipelines and is particularly concerned with theinstallation of offshore pipelines from floating vessels.

2. Description of the Prior Art A number of methods have been developedfor the laying of underwater pipelines in recent years. These include(1) the bottom-pull method wherein the line is fabricated on shore,launched into the water, and then pulled into position along the bottomby means of winches mounted on a barge or the opposite shore; (2) theflotation method in which long strings of pipe fabricated ashore aretowed to the site on floats or pontoons, connected by lifting adjacentends out of the water with the cranes on a tie-in barge, and thenlowered into place by systematically removing the floats or pontoons;(3) the reeled pipe method in which the pipe is unwound from a largereel mounted on a barge and lowered into place on the bottom; and (4)the lay barge method wherein individual sections of pipe are weldedtogether on a barge and then lowered into position over a stinger at thestern as the barge advances. Each of these methods has limitations whichrestrict its application. It has been found that the bottom-pull methodis generally practical over only short distances; that the flotationmethod is difficult to control and may result in buckling or the loss oflong pipe sections at sea; and that the reeled pipe method is normallyrestricted to pipe of relatively small diameter. Most operationsrequiring the laying of relatively large diameter pipe in the open seahave therefore been carried out by using the lay barge method.

The lay barge method normally requires the use of a barge provided withmultiple work stations arranged to permit handling of the pipe in anassembly line manner. Each section of pipe is welded into place, theweld is inspected, and the field joint is coated before it moves downthe stinger at the stern of the barge. The stinger at least partiallysupports the line between the barge and the ocean floor to prevent thegeneration of excessive bending stresses. Wheel or track-type tensioningdevices on the barge hold the pipe in tension and thus further aid incontrolling stresses. Experience has shown that this method is generallyslow because of the time consuming pipe handling and welding operationscarried out on board the barge, that it is expensive because of the longperiods over which marine equipment must be made available, and that itis difficult to use in deep water because of limitations on the amountof tension that can be applied to the pipe as it is lowered into place.Costly delays are often necessary because of changes in the weatherbefore a job can be completed. Efforts to overcome these and relateddisadvantages have in the past been only partially successful.

SUMMARY OF THE INVENTION The present invention provides an improvedsystem for laying underwater pipelines which avoids many of the problemsencountered heretofore and generally permits much better control of thelowering operation and faster pipelaying than would otherwise bepossible. The method involves the use of a long preassembled floatingpipeline section which extends in front of a lay barge and is lowered tothe sea floor from the stern of the barge as it advances. The suspendedportion of the pipeline between the sea floor and the end of thefloating section is maintained in tension by a second vessel positionedin front of the barge. When the lay barge reaches the end of thefloating section, tension is transferred to the lay barge by clampingthe pipe in a fixed position and the second vessel then moves off.Another section of pipe is connected in place by welding the endstogether on board the barge, tension is applied to the new section bythe second vessel, tension is released by the lay barge, and laying ofthe line is resumed. This eliminates the necessity for storing andhandling individual pipe sections on board the lay barge; reduces thenumber of welds or other connections that must be made at sea;simplifies inspection, testing and coating of the pipe; permits theapplication of tension to the pipeline without relying on conventionaltensioning devices; makes possible the laying of pipelines in deeperwater than would otherwise be feasible; facilitates the laying ofpipelines over trenches that are difficult to traverse by conventionalmethods; simplifies mooring problems during pipelaying operations; andhas other advantages.

In carrying out the invention, long preassembled pipeline sections arefirst made up on shore or in shallow water and are capped or plugged attheir ends. Buoys may be attached to help support the sections ifnecessary. These long floating sections are then towed to the pipelinesite by one or more tugs or other vessels. A lay barge to be used ininstalling the line will normally be moored at the site near an offshoreplatform or other installation from which the line is to extend. Thefirst floating pipeline section is aligned with this barge so that itextends outwardly in front of the barge. One end of this first sectionis pulled onto a pipe ramp or similar device at the bow of the lay bargewhile a tug or pulling barge holds the other end of the floating sectionin tension and maintains its alignment with the barge. If necessary,tugs or barges can also be stationed at intermediate points to helpmaintain alignment. Traction on the line is continued by means ofwinches on the lay barge until the end of the line extends over thebarge to a stinger at its stern. After the end of the pipeline has beenlowered to the ocean floor, it is secured to the platform or bottom inthe conventional manner.

Once the initial pipeline section is in position on the lay barge andarrangements have been made for securing the end of the section at thesea floor, laying of the line may be commenced. The floating end of theline is held in tension by the tug or other vessel positioned in frontof the lay barge as the barge moves forward along the floating pipelinesection, lowering the line into place behind it. When the lay bargereaches the end of the first section, it takes over the tension from thetug or pulling barge. This is done by clamping the line in place on thelay barge pipe ramp so that it cannot move with respect to the barge. Asecond floating pipeline section which has been towed from shore orshallow water and aligned with the lay barge is then pulled up the piperamp at the bow of the barge and the ends of the two sections are weldedor mechanically connected together. After the connection has been madeand the joint has been inspected and coated, tension is transferred tothe tug or crane barge and the lay barge again moves forward, loweringthe pipeline into place over its stern. This procedure is continueduntil the entire line has been laid.

As pointed out earlier, the method of the invention substantiallyreduces the cost of installing offshore pipelines. Marine equipmentrequired for the pipelaying operation is often needed for much shorterperiods of time than in conventional operations and hence verysignificant savings are possible. In some cases it also permitspipelaying operations to be carried out in deep water where conventionalmethods are ordinarily not feasible. These and other advantages of themethod make it attractive for use under a wide variety of conditions. Itis particularly useful in areas such as the Artic Ocean and the NorthSea where the working seasons are short and periods of good weather maylast for only a few days at a time.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 in the drawing illustrates aninitial stage in the installation of a light weight offshore pipeling inaccordance the invention;

FIG. 2 is a top view of a lay barge suitable for use in carrying out themethod shown in FIG. 1;

FIG. 3 is a longitudinal view of the barge of FIG. 2;

FIG. 4 illustrates an early stage in the laying of a negatively buoyantpipeline by the method of the invention; and

FIG. 5 represents a later stage in the operation shown in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIIVIENTS The apparatus depicted inFIG. 1 of the drawing includes a long preassembled pipeline sectionwhich has been fabricated on shore or in shallow water, capped orplugged at each end to prevent the entry of water, and towed out to thepipeline location by one or more tugs not shown in the drawing. The pipeused in fabricating section 10, which may be of any desired diameter, iswelded together or mechanically connected by means of threaded joints,inspected for the detection of leaks, and then given an external coatingof an asphaltic material or other agent for the prevention of corrosion.An internal protective coating and an outer sheath of concrete orsimilar weighting agents sufficient to give the desired specific gravitymay also be provided if needed. The pipe should be sufficiently light tofloat at the surface 11 of the water when it is filled with air andhence heavily weighted pipe will not normally be used. The length ofeach pipeline section will depend in part on the size of the pipe used,the sea conditions, and the distance over which it is to be towed butwill normally range from about 300 feet to about 3,000 feet or more.Generally speaking, the lengths should be the maximum compatible withwave height, current velocities, and equipment capabilities. One of theadvantages of the invention is that these long preassembled sections canbe made up in an assembly line operation at a protected onshore base andthat several long sections can be prepared simultaneously if desired.This use of an onshore base permits fabrication much more rapidly and atconsiderably less expense than would be incurred if the pipelinesections were made up at sea.

A lay barge 12 is positioned at the pipeline construction site. Thisbarge, shown in greater detail in FIGS. 2 and 3 of the drawing, includesan elongated barge hull 13 containing a hold in which supplies, fuel andequipment may be stored. A superstructure 14 including control equipmentand the like is positioned on the deck of the barge as shown. The bargeemployed may be moved by a spread mooring system technique using winches15 or may be provided with a propulsion system which does not appear inthe drawing. Such a system will normally include a propeller at the bowand stern of the vessel for providing forward thrust and two or morelaterally directed propellers near the bow and stem for maintaining thedesired heading or changing the lateral position of the barge asnecessary. These side thrusters may be operated selectively to permitprecise control of the vessels position under various wind, wave andcurrent conditions.

On one side of the barge is a double pipe ramp 16 which slopesdownwardly from points near the middle of the barge to points near thewater level at the bow and stem. The ramp is provided with rollers 17 orother means for guiding and supporting the pipeline section I 10 as itmoves over the ramp. A working area 18 is located amidships between thebow and stem portions of the pipe ramp to permit the welding ormechanical connection of adjacent pipeline sections to one another andthe inspection and coating of the joints. The floor of the working areamay be recessed with respect to the adjacent deck as shown to permitready access to the full pipe circumference. In lieu of this, the piperamp may be made higher so that the working area is flush with the deck.Hydraulically or mechanically actuated pipe supports 19 and 20 arelocated within the working area. These supports, which may be ofconventional design, are fitted with rollers on which the pipe rests andwith clamps 21 and 22 for holding it in position. Each of the supportscan be adjusted to permit precise alignment and abutment of the ends oftwo adjacent pipe sections. Davits 23, 24, 25, 26 and 27 are located atintervals along the deck above the pipe ramp to facilitate handling andpositioning of the pipe as required. Pipe handling equipment of thistype is described in the literature in connection with lay barges ofconventional design and will therefore be familiar to those skilled inthe art. The barge may be used in conventional pipelaying operations byinstalling a temporary extension of the deck over the forward end of thepipe ramp so that pipe sections can be moved from the deck onto theramp.

The lay barge 12 may also be equipped with a pipe tensioning device 28of conventional design if desired. Such a device is not normallyemployed in carrying out the improved method but may be provided for usein conventional operations. If provided, this device will normallyinclude tracks or wheels which engage the pipe and generate friction torestrict its motion relative to the barge. The tensioning device willnormally be located near the stern of the barge between work station 18and stinger 29. The stinger is an elongated buoyant member provided withrollers or similar members 30 which is hinged to the barge and supportsthe pipe as it is lowered to the sea floor. Any of a wide variety ofstingers of conventional design may be employed. Only a portion of thestinger is shown in FIGS. 2 and 3. A crane 31 and other conventionalequipment not shown in the drawing may also be provided to facilitatethe movement of pipeline sections onto and off of the barge. It will beunderstood that the method of the invention is not restricted to use ofthe particular barge shown and that barges fitted with outrigger devicesor other means in lieu of a double pipe ramp can be employed if desired.I

After the first preassembled long pipeline section has been towed to thepipeline construction site, it is positioned by the tugs used in thetowing operation so that it extends in the direction the pipeline is tobe laid with one end near the bow of the lay barge. One or more lines isthen extended from the lay barge to the near end of the pipeline sectionand the section is pulled onto the pipe ramp at the bow of the barge.Here the end of the pipeline section is connected to a line from a winchon the deck and the section is pulled up the ramp and onto the barge.The cap or plug is then removed from the end of the pipeline section.

Concurrently, a pulling barge 34, a tug, or a similar vessel ispositioned near the other end of the floating pipeline section. A line35 from the pulling barge is attached to the end of the pipeline sectionand the end is lifted clear of the water. The cap or plug in this end ofthe pipeline section is then also removed. The purpose in raising theend of the-section and removing the cap or plug is to permit the escapeof air from the pipe and thus permit the entry of water. In lieu oflifting the pipe, the end may be connected to a buoy-supported hose orthe like. Tension is applied to the floating line by means of a winch 36on the barge to maintain alignment with the lay barge. At this point,assuming that the pipeline is to extend from an offshore platform 37 orsimilar installation near the lay barge, a cable may be run from a winchon the deck of the barge to an underwater sheave on the platform andbrought back to the end of the pipeline section to assist in positioningthe end of the section when it reaches the sea floor. The pipe is thenlowered over stinger 29 and pulled into position adjacent the base ofthe platform by means of the winches on the lay barge.

As the pipeline section enters the water at the stern of the barge, thewater rises within it so that the level is substantially the same as theoutside water level. Since the suspended end of the pipe is thus filledwith water, it is considerably heavier than most pipelines installedfrom conventional lay barges. For this reason, the stinger shown in thisparticular embodiment of the invention is designed to support thepipeline between the lay barge and a point near the sea floor. Ifdesired, the stinger may be provided with buoyant members to further aidin supporting the line and in shallow water may be designed to dragalong the bottom. The design considerations for stingers are well knownto those skilled in the art and need not be set forth in detail topermit an understanding of the invention. The tension maintained on theline by pulling barge or similar vessel 34 after the end has beensecured to the platform or sea floor reduces the bending stresses in theline between the end of the stinger and the sea floor and hence oftenpermits the use of a somewhat shorter stinger than might otherwise berequired.

After the open end of the pipeline has been positioned adjacent the baseof the platform or similar structure 37, it may be anchored to thebottom or tied to the platform if desired. Once the end is in place, thelay barge begins to move forward in the water toward pulling barge 34while the pipeline section moves across the lay barge ramp, down thestinger 29 and onto the bottom. The lay barge may be propelled by meansof its mooring system or dynamic positioning equipment or by one or moretugs. Tension is maintained at the floating end of the line by means ofa winch on the pulling barge. This forward movement of the lay barge andlaying of the line is continued until the lay barge reaches a point nearthe pulling barge. The suspended end of pipeline section 10 is thenlowered into the water by the crane on barge 34. The lay barge movesforward until the end is in position on the barge and is ready for theaddition of another pipeline section. Tension is maintained by thepulling barge until the end has been secured by pipe clamp 21. Thismaintains the necessary tension and thus permits release of the pipelineby the pulling barge. The pulling barge then moves forward to a pointabout one pipeline section length in advance of the lay barge.

After the first pipeline section has been positioned as described above,a second pipeline section which has been towed to the construction sitefrom the shore base is brought up by tugs to a position in front of thelay barge. This section, like the previous one, is capped or plugged atboth ends so that it will float. A line from the lay barge is connectedto the near end of the second section and the end is pulled up the piperamp onto the barge into a position where it can be clamped in placeadjacent working area 18. At the same time, a line from the pullingbarge or other vessel is affixed to the other end of the second sectionand the end is raised from the water. The caps or plugs are then removedfrom both ends of the second pipeline section.

The ends of the two pipeline sections on the lay barge are aligned byadjusting pipe supports 19 and 20 and clamping them in place. The twoends can then be welded or mechanically connected together. In theinterest of speed, it is generally preferred that the sections be joinedby means of automatic welding equipment, but other means may be used ifdesired. Once the connection has been made, the pulling barge appliestension and clamps 21 and 22 on the lay barge are released. The laybarge once more begins to move forward toward the pulling barge. As thebarge moves, the pipe is lowered to the ocean floor at the stern of thebarge. Because of the long pipeline sections used and the relativelysmall number of joints that have to be made up at sea, laying of theline normally progresses very rapidly. The marine equipment is generallyrequired for a much shorter period of time than would be the case ifindividual sections of pipe were welded together aboard the lay bargeand then installed. The use of a pulling barge or similar vessel toapply tension to the line as it is laid permits the application of muchgreater force than can generally be obtained with the conventionalconstant tension restraining devices that are used on barges where thepipe is carried aboard the barge and made up one joint at a time. Theuse of a pulling barge for the application of tension permits operationsin much deeper water than would otherwise be possible.

The pipeline installed as described above can be used for thetransmission of crude oil or other liquids but will normally not besatisfactory as a gas line unless it is anchored to the sea bott'orn.Because of the low weight, the line will tend to rise if filled withgas. Anchors can be employed if desired but it isgenerally preferred toemploy an alternate method where the installation of a heavier line tobe used for gas transmission purposes is necessary or where the linemust be laid in deep water.

FIGS. 4'and 5 of the drawing illustrate an alternate embodiment of theinvention suitable for the laying of heavier pipe in relatively deepwater. The area depicted in FIGS. 4 and 5 is one in which the pipelineroute crosses an ocean floor trench which may be 5 miles or more inwidth and from 1,500 to 2,000 feet'or more in depth. Such trenches,common in many areas, present severe problems in conventional pipelayingoperations because of difficulty in mooring the lay barge in deep waterand because of limitations on the amount of tension that can be appliedto the line with conventional track or wheel type tension devices. Atpresent, such devices can seldom be used to apply more than about150,000 lbs and in many cases this is insufficient to permit laying ofthe line in water more than a few hundred feet deep.

The method shown in FIGS. 4 and 5 is similar to that described earlierin that it is carried out by first constructing long preassembledpipeline sections 40 at a shore base where assembly line equipment isavailable. Each pipeline section will again be from about 300 feet toabout 3,000 feet or more in length. The pipe may be of any desireddiameter. Each pipeline section is capped or plugged at the ends toprevent the entry of water. The pipe employed is sufficiently heavy thatit will tend to sink even though filled with air and hence buoys 41 areattached at intervals along each section. Any of a variety of differenttypes of buoys may be used. The spacing of the buoys will depend uponthe size of the individual buoys and the size and weight of the pipeemployed. Each pipeline section should be as long as possible,considering wave heights, current velocities, and equipmentcapabilities. Two or more tugs or other vessels will normally be used totow each pipeline section to the construction site.

A lay barge 42 similar to that shown in FIGS. 2 and 3 of the drawing maybe used in carrying out the method depicted in FIGS. 4 and 5. This bargewill normally be fitted with a stinger 43 but the use of such a stingeris not always essential. External floats or other means can be used tosupport the line without a stinger if desired.

After the first pipeline section has been towed to the constructionsite, it is positioned with one end near the front of lay barge 42 andthe other end extending along the pipeline route in front of the barge.A line 44 extends from the forward end of the section to a winch 45 onpulling barge 46 to assist in maintaining the floating section in theproper position. The pulling barge is moored by means of lines 47 and48. Once the floating pipeline section is in position, one end is pulledup the forward end of the pipe ramp on the lay barge. Lines extendingfrom winches on the deck of the barge may be used for this purpose. Theother end is held with sufficient tension to maintain alignment of thepipeline section in front of the lay barge. The buoys attached to thepipeline section are removed as the section comes aboard the lay bargeand are stored for return to the shore base. The lay barge then beginsto move ahead while pushing the capped or plugged end of the pipelinedown the stinger at the stern of the barge. The air contained in thepipeline section will contribute buoyancy and thus aid in supporting thesection between the barge and the ocean floor. Once the end of thepipeline reaches the sea bottom and is secured to the bottom or aplatform, sufficient tension must be applied by the pulling barge at theother endto keep the pipe supported and prevent excessive bending. Thetension needed under particular operating conditions can be calculated.If the end of the pipeline is to be located near a platform or similarinstallation, it can be positioned adjacent the platform by running acable from a winch on the lay barge to a pulley attached to the base ofthe platform and back to a padeye on the end of the pipeline. This willpermit pulling of the end of the pipeline section to the platform ifnecessary. Other means for securing the end of the first pipelinesecfion include the use of anchors and pilings. The tension required toprevent excessive bending of the line can be supplied by means of apulling barge or tug as shown.

After laying of the pipeline has been started as described above, thelay barge continues to move forward along the floating pipeline section,removing buoys, until it reaches a point near the end of the floatingsection. The pulling barge maintains tension in the pipeline until ithas been secured by clamp 21 on the lay barge. The pulling barge thenreleases the line and moves forward to receive the next pipelinesection. The cap or plug on the end of the line is then removed. Sincethe line is capped at the bottom, it remains filled with air. By thistime, a second pipeline section has been towed into position in front ofthe barge. A line is attached to the end of the second section and it ispulled up the pipe ramp at the bow of the lay barge by means of wincheson the barges. The cap or plug is removed from the end of the secondsection and the two sections are aligned. Tension is maintained in thesecond pipeline section by means of a line extended from the pullingbarge to the forward end of the section. The two pipeline sections arethen welded or mechanically connected together. Once this has been done,the lay barge begins to move forward again while the pulling bargemaintains tension in the pipeline. The buoys attached to the floatingline in front of the barge are removed as the line is lifted from thewater onto the barge. These buoys are returned to the shore base for usein supporting additional pipeline sections as they are moved from thebase to the construction site.

The method of FIGS. 4 and 5 permits the crossing of deep trenches suchas that shown without the difficulties encountered in conventionalpipelaying operations. In FIG. 4, the pulling barge is moored on the farside of the trench and the floating pipeline extends across the trenchto the lay barge. Behind the lay barge, the pipeline extends down thestinger and onto the ocean floor. Since the pulling barge maintains theline in tension, the lay barge can be moved forward by towing it alongthe floating pipeline. This eliminates the necessity for using mooringlines to move the lay barge and avoids difficulties normally associatedwith the placement of anchors in very deep water. As much as 400,000 lbsof tension can be applied to the pipeline as it extends down the stingerto the sea floor at the bottom of the trench and hence excessive bendingof the pipe can be avoided. Once the line has been laid across thetrench and the lay barge has reached the other side as shown in FIG. 5,the amount of tension applied to the line can be reduced and operationscan be continued as described earlier.

I claim:

1. A method for laying an underwater pipeline from a first floatingvessel as said vessel moves forward along the pipeline route whichcomprises taking one end of an elongated floating pipeline extendingalong said route aboard said vessel at a point near the bow thereof,reducing the buoyancy of the pipeline adjacent said vessel as the vesseladvances along said route, and lowering the pipeline to the bottom ofthe body of water from a point near the stern of the advancing vesselwhile maintaining the line in tension by means of a second floatingvessel to which the other end of said floating pipeline is secured.

2. A method as defined by claim 1 wherein said floating pipeline isfilled with air and the buoyancy is reduced by admitting water into theline as said line is lowered behind said first floating vessel.

3. A method as defined by claim 1 wherein said floating pipeline issupported by floats positioned at intervals along the line between saidfirst and second vessels and the buoyancy is reduced by removing floatsbefore the line is lowered to the bottom of the body of water.

4. A method as defined by claim 1 wherein said first floating vessel ismoved forward by towing said vessel along the floating pipeline.

5. A method for laying an underwater pipeline from a first floatingvessel as said vessel moves along the pipeline route which comprisespositioning an elongated floating pipeline section in front of saidfirst vessel with one end near the bow thereof and the other endextending forward along said route to a second floating vessel; takingsaid one end of said floating pipeline section aboard said firstfloating vessel while supporting the other end at said second floatingvessel; moving said first vessel forward beneath the pipeline section;reducing the buoyancy of said pipeline section at a point adjacent saidfirst vessel as said vessel advances; and lowering said pipeline sectionto the bottom of the body of water from a point near the stern of saidfirst vessel as said vessel moves forward along the pipeline route.

6. A method as defined by claim 5 wherein said pipeline section is heldin tension by said second vessel as the pipeline section is lowered tothe bottom of said body of water.

7. A method for laying an underwater pipeline from a floating vessel asthe vessel moves along the pipeline route which comprises positioning anelongated floating pipeline section which is closed at both ends infront of said vessel with one end near the bow thereof and the other endextending forward along said pipeline route; taking said one end of saidfloating pipeline section aboard said vessel near the bow thereof;supporting'said other end of said pipeline section at a point in advanceof said vessel along said pipeline route; opening said one end of saidpipeline section aboard said vessel and venting said other end to theatmosphere; moving said vessel forward while passing said pipelingsection over at least a portion of said vessel; and lowering saidpipeline section to the bottom of said body of water from a point nearthe stern of said vessel as the vessel advances.

8. A method as defined by claim 7 wherein said other end of saidfloating pipeline section is vented to the atmosphere by lifting saidend from the water and open ing said end.

9. A method for laying an underwater pipeline from a first floatingvessel as said vessel moves along the pipeline route which comprisespositioning an elongated preassembled pipeline section in front of saidvessel with one end near the bow thereof and the other end extendingforward along the pipeline'route, said pipeline section being supportednear the surface of the water by buoyant members attached at pointsalong the pipeline section; taking one end of said pipeline sectionaboard said vessel near the bow thereof; holding said other end of saidpipeline section in place by means of a second vessel moored in advanceof said first vessel along the pipeline route; moving said first vesselforward along the pipeline route while passing said pipeline sectionover at least a portion of said first vessel; removing at least part ofsaid buoyant members from said pipeline section at a point adjacent saidfirst vessel; and lowering said pipeline section to the bottom of saidbody of water astem of said first vessel as the vessel advances whilemaintaining said pipeline section in tension by means of said secondvessel.

10. A method for laying an underwater pipeline from a first floatingvessel as said vessel advances along the pipeline route which comprisespositioning preassembled floating pipeline sections along said route inadvance of said vessel; passing each such section in turn over saidvessel as the vessel advances; connecting adjacent sections to oneanother aboard said vessel; changing the buoyancy of each section sothat it will remain submerged; and lowering the pipeline onto the bottomof the body of water astern of said first vessel as said vessel advanceswhile maintaining the pipeline in tension by means of a second floatingvessel moored in advance of said first floating vessel.

11. Apparatus for laying a submerged pipeline on the bottom of a body ofwater which comprises a buoyant vessel having a deck, a bow, and astern; guide means on said vessel for guiding an elongated string ofpipe upwardly out of the water near said bow of said vessel, along saidvessel adjacent said deck, and downwardly into the water again near saidstern of said vessel; means for aligning the end of one pipe stringextending from said vessel into the water behind the vessel with the endof an adjacent pipe string extending from the water in front of saidvessel onto said vessel; means for connecting together adjacent pipestrings aligned endto-end on said guide means; and means near the sternof said vessel for lowering a pipe string from said guide means to thebottom of the water astem of said vessel.

12. Apparatus as defined by claim 11 wherein said guide means comprisesa pipe ramp extending upwardly from a point adjacent the water levelnear the bow of said vessel to a point astem of said how, along saidvessel adjacent said deck, and downwardly from a point forward of saidstem to a point adjacent the water level near the stern of said vessel.

1. A method for laying an underwater pipeline from a first floatingvessel as said vessel moves forward along the pipeline route whichcomprises taking one end of an elongated floating pipeline extendingalong said route aboard said vessel at a point near the bow thereof,reducing the buoyancy of the pipeline adjacent said vessel as the vesseladvances along said route, and lowering the pipeline to the bottom ofthe body of water from a point near the stern of the advancing vesselwhile maintaining the line in tension by means of a second floatingvessel to which the other end of said floating pipeline is secured.
 2. Amethod as defined by claim 1 wherein said floating pipeline is filledwith air and the buoyancy is reduced by admitting water into the line assaid line is lowered behind said first floating vessel.
 3. A method asdefined by claim 1 wherein said floating pipeline is supported by floatspositioned at intervals along the line between said first and secondvessels and the buoyancy is reduced by removing floats before the lineis lowered to the bottom of the body of water.
 4. A method as defined byclaim 1 wherein said first floating vessel is moved forward by towingsaid vessel along the floating pipeline.
 5. A method for laying anunderwater pipeline from a first floating vessel as said vessel movesalong the pipeline route which comprises positioning an elongatedfloating pipeline section in front of said first vessel with one endnear the bow thereof and the other end extending forward along saidroute to a second floating vessel; taking said one end of said floatingpipeline section aboard said first floating vessel while supporting theother end at said second floating vessel; moving said first vesselforward beneath the pipeline section; reducing the buoyancy of saidpipeline section at a point adjacent said first vessel as said vesseladvances; and lowering said pipeline section to the bottom of the bodyof water from a point near the stern of said first vessel as said vesselmoves forward along the pipeline route.
 6. A method as defined by claim5 wherein said pipeline section is held in tension by said second vesselas the pipeline section is lowered to the bottom of said body of water.7. A method for laying an underwater pipeline from a floating vessel asthe vessel moves along the pipeline route which comprises positioning anelongated floating pipeline section which is closed at both ends infront of said vessel with one end near the bow thereof and the other endextending forward along said pipeline route; taking said one end of saidfloating pipeline section aboard said vessel near the bow thereof;supporting said other end of said pipeline section at a point in advanceof said vessel along said pipeline route; opening said one end of saidpipeline section aboard said vessel and venting said other end to theatmosphere; moving said vessel forward while passing said pipelingsection over at least a portion of said vessel; and lowering saidpipeline section to the bottom of said body of water from a point nearthe stern of said vessel as the vessel advances.
 8. A method as definedby claim 7 wherein said other end of said floating pipeline section isvented to the atmosphere by lifting said end from the water and openingsaid end.
 9. A method for laying an underwater pipeline from a firstfloating vessel as said vessel moves along the pipeline route whichcomprises positioning an elongated preassembled pipeline section infront of said vessel with one end near the bow thereof and the other endextending forward along the pipeline route, said pipeline section beingsupported near the surface of the water by buoyant members attached atpoints along the pipeline section; taking one end of said pipelinesection aboard said vessel near the bow thereof; holding said other endof said pipeline section in place by means of a second vessel moored inadvance of said first vessel along the pipeline route; moving said firstvessel forward along the pipeline route while passing said pipelinesection over at least a portion of said first vessel; removing at leastpart of said buoyant members from said pipeline section at a pointadjacent said first vessel; and lowering said pipeline section to thebottom of said body of water astern of said first vessel as the vesseladvances while maintaining said pipeline section in tension by means ofsaid second vessel.
 10. A method for laying an underwater pipeline froma first floating vessel as said vessel advances along the pipeline routewhich comprises positioning preassembled floating pipeline sectionsalong said route in advance of said vessel; passing each such section inturn over said vessel as the vessel advances; connecting adjacentsections to one another aboard said vessel; changing the buoyancy ofeach section so that it will remain submerged; and lowering the pipelineonto the bottom of the body of water astern of said first vessel as saidvessel advances while maintaining the pipeline in tension by means of asecond floating vessel moored in advance of said first floating vessel.11. Apparatus for laying a submerged pipeline on the bottom of a body ofwater which comprises a buoyant vessel having a deck, a bow, and astern; guide means on said vessel for guiding an elongated string ofpipe upwardly out of the water near said bow of said vessel, along saidvessel adjacent said deck, and downwardly into the water again near saidstern of said vessel; means for aligning the end of one pipe stringextending from said vessel into the water behind the vessel with the endof an adjacent pipe string extending from the water in front of saidvessel onto said vessel; means for connecting together adjacent pipestrings aligned end-to-end on said guide means; and means near the sternof said vessel for lowering a pipe string from said guide means to thebottom of the water astern of said vessel.
 12. Apparatus as defined byclaim 11 wherein said guide means comprises a pipe ramp extendingupwardly from a point adjacent the water level near the bow of saidvessel to a point astern of said bow, along said vessel adjacent saiddeck, and downwardly from a point forward of said stern to a pointadjacent the water level near the stern of said vessel.